The present invention relates generally to devices and processes for strengthening unreinforced masonry structures against seismic-induced failure. More specifically, the present invention relates to a conical seismic anchor and drill bit, and related processes for anchoring wood and floor roof diaphragms/joists to unreinforced masonry walls to increase seismic safety in older buildings.
One of the first and most effective steps taken to make old unreinforced masonry buildings more seismically resistant is the anchoring of wood floor and roof diaphragms/joists to unreinforced masonry walls. Unanchored walls and diaphragms can separate during a quake, resulting in collapse of one of both and needless deaths and destruction. Anchors keep walls and diaphragms tied together and reduce the chance of failure. Anchoring systems also spread the effects of earthquake forces over more of the structure and from walls perpendicular to seismic acceleration to walls parallel to the acceleration. This allows seismic forces to be dissipated by more of the structure and by parts most able to accept such force (walls parallel to seismic acceleration).
Three anchoring types commonly in use are tension anchors, shear anchors and combination anchors. The type used depends on the specific application, engineering requirements, and the local building codes.
Tension anchors are used to keep walls which are perpendicularly oriented to seismic acceleration from pulling away from floor and roof diaphragms/joists. When not anchored, not only are the walls more likely to pull away from the diaphragms, causing the diaphragms to drop, but the unrestrained unreinforced masonry wall itself can over-extend and collapse. A simple tension anchor is typically a threaded shaft placed within a hole drilled perpendicularly completely through the unreinforced masonry wall, with washers and nuts applied to both ends of the threaded shaft. Brackets and the like are fixed to one end of the threaded shaft and, in turn, connected to floor/roof joists to anchor the unreinforced masonry wall to the joists.
Shear anchors help prevent diaphragms/joists from sliding inside unreinforced masonry walls and also help to transfer seismic forces from the walls to the diaphragms and vice versa. Walls perpendicular to ground acceleration transfer force into anchored diaphragms and then into walls parallel to acceleration through this type of anchor. Shear anchors accept lateral forces, in contrast with the parallel forces accepted by tension anchors.
A shear anchor is not a through bolt. Typically a shear anchor comprises a cylindrical cavity bored at least eight inches into but not completely through the unreinforced masonry wall. Either dry pack mortar or chemical epoxy compounds are used to secure a threaded shaft within the cavity. When dry pack mortar is used, the cavity is usually two and a half inches in diameter. When epoxy compounds are used, the cavity is typically one inch in diameter.
Combination-type anchors combine the functions of both tension and shear anchors into one unit. There are three common types of combination anchors, all of which are mortar or epoxy packed to accept lateral forces like a shear anchor. One type of combination anchor uses a through bolt for tension anchoring. The other two types of combination anchors are angled downwardly from the wall face at 221/2.degree. for tensioning where through bolts are impractical. One such 221/2.degree. anchor utilizes a beveled washer to square the anchor with the wall face, while the other employs a pre-bent anchor bolt.
An excellent publication illustrating prior seismic anchors is The City of Los Angeles Department of Building and Safety, Building Bureau, Earthquake Safety Division Guideline 3 entitled "Torque Testing And Continuous Inspection For Grouted And Chemical, Combination, Shear And Tension Anchors (Rev-890809)" This publication, incorporated herein by reference, illustrates primarily shear and combination seismic anchors, and requirements for testing grouted and chemical anchors as required by .sctn.91.8809(f) of the Los Angeles Municipal Code (Earthquake Hazard Ordinance).
A problem encountered with prior seismic anchors is that there exists the possibility that the anchor was improperly installed into the unreinforced masonry wall. Both the chemical anchors and the grouted anchors will fail during a seismic event if the hand-applied chemical filler or non-shrink grout between the threaded bolt and the cylindrical cavity is improperly placed within the cavity. This has necessitated the testing procedure set forth in Guideline 3 mentioned above.
Concrete anchors are not suitable for use in anchoring unreinforced masonry structures against seismic forces. Typical concrete anchors include an element forced into a pre-drilled cavity and caused to expand against the walls of the cavity. Such an anchor, if utilized in unreinforced masonry structures, would simply cause the walls of the pre-drilled cavity to crumble into dust or fail.
Accordingly, there has been a need for a novel seismic anchor and process for attaching the anchor to unreinforced masonry structures, which meets minimum building code strength requirements and yet eliminates, after installation, the need to perform in-place torque, tension and shear testing. Additionally, a novel seismic anchor and installation process is needed which eliminates field mixing requirements, and has a long shelf life without special storage requirements. Moreover, such a seismic anchor and related process is needed which retains structural strength at 350.degree. F., requires no setting time in order to complete installation, and attains maximum strength instantly upon assembly to the unreinforced masonry wall. Further, a novel seismic anchor is needed which can be simply replaced, if necessary, reused, and/or retightened after a small earthquake. The present invention fulfills these needs and provides other related advantages.